Electron Dot Diagrams

The last thing that I need to teach you about bonding is a handy way to represent the bonds in a diagram. Using electron dot diagrams.

The premise of electron dot diagrams is simple. Write the element's symbol, and draw the valence electrons as dots surrounding the symbol.

Simple enough, isn't it?

Now, when you're drawing covalent bonds, draw the electrons that are being shared between the two atoms and the electrons that aren't being shared around one atom only.

The two oxygen atoms share 2 electrons each in a double bond. These electrons are drawn in between the two atoms. The remaining electrons are drawn surrounding the individual atoms. The same thing goes for nitrogen except that there's a triple bond there. The water and carbon dioxide show the same things except with different atoms.

In some diagrams, they use dots and crosses to show which electrons belong to which atoms. The above examples could be drawn like this:

It doesn't really matter which atoms get the dots and which atoms get the crosses. The dots and crosses are only there for clarification.

Most times it's pretty easy to tell where the bonds go and how many bonds you need, but if it isn't, there's a nifty little formula that you can use to work out how many bonds you need. From there, it's a lot easier to work out where they need to go.

(Electrons wanted - electrons you have)/2

For example, carbon dioxide wants 24 electrons. (The C atom wants to have 8 electrons and the O atoms want 8 each.) However, you only have 16 electrons (6 per oxygen, 4 for carbon). (24-16)/2 = 4. Four bonds are required- a double bond between C and one of the O atoms, and a double bond between C and the other O atom.

Let's try it with water this time. Water wants 12 electrons (2 per hydrogen and 8 for oxygen). Water only has 8 electrons (1 per hydrogen and 6 for oxygen). (12-8)/2 = 2 bonds. From there, it's easy enough to work out where they go.

For ions, the electron diagrams are pretty much the same. Just remember that in the case of positive ions, you have fewer electrons, and with negative ions, you have more electrons. If you're using the dot-and-cross type of diagram, you can draw in the extra electrons in negative ions using an open circle.

Drawing diagrams of ionic bonds is the next step. In ionic bonds, you draw the ions separately. Each ion is surrounded by brackets, and the + or - is written as a superscript. To indicate how many of each ion you need, you write 2 x or 3 x or whatever before the bracket... I think...

As you can hopefully see, the second example, ammonium carbonate, contains both covalent and ionic bonds. The N and H atoms in the ammonium ion are covalently bonded together, as are the C and O atoms in the carbonate ion, but the two ions are held together by ionic bonds.

In the ammonium ion, you might notice that one of the bonds between N and H has two electrons that come from the same atom. There's a special name for this- a dative covalent bond or a coordinate bond. Not really important for us to know at this stage, but hey, nothing wrong with knowing some more big words!

That's all I really have to say on electron dot diagrams. As far as I know, you can't really draw a dot diagram for metallic bonding. Feel free to correct me on that one.